1. Field of the Invention.
This invention pertains to communication systems in which different stations are selectively connected to each other by frequency selection, particularly such systems in which a satellite-borne relay is employed; and it deals specifically with improvement in the art by which more than two stations are commonly connected.
2. Description of the Prior Art.
U.S. Pat. No. 3,842,350, of Gross, assigned to the assignee of this application, is incorporated herein by reference. The Abstract of this patent reads as follows:
"Ground terminal serving a number of individual telephone lines in a local region converts their signals to an intermediate frequency without regard to whether called station is in same local region or in remote region linked by relay satellite. Switching equipment routes remote region calls to up-frequency converter link to satellite, and local region calls to intermediate-frequency bus for connection to local region called line. Intermediate-frequency switching system thus replaces conventional combination of standard telephone exchange for local region calls plus separate switching system for ground link to relay satellite, with economy in equipment for appropriate mix of local and remote traffic."
According to the disclosure of that patent, each subscriber station (or trunk) is provided with a trunk access unit (abbreviated for convenience to its acronym TAU) which comprises a modulator and a transmitting IF oscillator whose frequency may be adjusted by control signals from a routing controller, and a demodula tor with a receiving IF oscillator whose frequency may also be adjusted by control signals from a routing controller. Thus each subscriber station may be assigned a transmitting intermediate frequency, and a different receiving intermediate frequency; complementary frequency assignments to a second subscriber station will effectively connect the two subscriber stations together, via a common channel which may include a satellite-borne relay. It is to be noted that only a single receiving channel is provided for each TAU, although this may be tuned to any frequency in its design range.
U.S. Pat. No. 3,617,644, of Boag, teaches a demand-assigned system employing a satellite link; it may best be described by its own abstract:
"Communication paths between a plurality of earth stations on a demand assignment basis are provided. Each participating station transmits a carrier that is received by all other stations and identifies the transmitting station. A remote station is called by transmitting the remote station calling frequency or address via the local carrier. The remote station then detects its address frequency on the local station carrier and automatically locks onto the channel carrying signals received from the local station. The remote station then transmits the signal frequency or address of the local station via the remote station carrier. When the address is received at the local station, it automatically locks onto the channel carrying signals received from the remote station, thereby providing a communication circuit (two-way communication path) between the calling and the called stations. Each participating station sends out a busy signal during the entire time it is in use. The busy signal from any one station is received by all other stations and prevents the calling of a busy station." His specification is directed to communication between exchanges, rather than individual subscriber telephones; and he states specifically (column 1, lines 55 through 58) "During the time in which the circuit interconnection is provided between a pair of exchanges they cannot communicate with any other exchanges in the pool." U.S. Pat. No. 3,110,773, of Miller, attacks the problem of providing service to incoming calls in order of their seniority, with further provision for giving priority to certain designated calls. However, each ground station has certain frequencies transmitting to a relay on a satellite permanently assigned to it; and it receives all communications relayed from the satellite except its own. It does not employ switching by frequency assignment; it aims at solving the priority or queuing problem which it assumes to exist because of the limited number of satellite channels available to any given station. It does indicate an operator position which permits the operator to communicate with any subscriber line but by conventional land-line connection to the subscriber.
U.S. Pat. No. 3,564,147, of Puente et al., discloses a way of permitting plurality of ground stations to select available relay channels. However, it is stated at column 3, lines 73 through 75: "The function of a telephone central and telephone centrals per se are well known in the art and they constitute the location and/or apparatus wherein calls are received and routed." It is thus evident that operator communication with any subscriber line is by conventional land-line connection; individual subscriber lines are not connected by frequency selection; only the exchanges are connected by satellite-borne relay.
U.S. Pat. No. time-division of Shimasaki et al., is concerned with a timedivision multiplex system in which a switching matrix on board a satellite switches information bursts occurring at different times to antennas appropriately directed to send each burst to the ground station for which it is intended. There is no suggestion of switching of individual subscriber lines by any means other than the obvious one of a standard telephone exchange connected to a ground station.
The preceding references, except for Gross cited at the beginning, are all that were found by a professional searcher. It is evident that none of them deals with a system in which individual subscriber lines are switched by frequency assignment, and hence none of them even encounters the problem of permitting a third party to intervene in a conversation between two subscribers connected together by frequency assignment as Gross teaches. Gross does not teach such intervention although he does teach switching by frequency assignment.